Bismuth oxide is conventionally prepared by one of the following methods: oxidizing fine powder bismuth by burning bismuth or injecting it into a heated zone (Patent Reference 1); dissolving bismuth metal in sodium nitrate by heating and passing chlorine gas (Patent Reference 2); neutralizing aqueous solution of bismuth nitrate with sodium hydroxide or potassium hydroxide, and precipitating the neutralized solution at 40-70° C. to obtain needle-crystalline bismuth oxide (Patent Reference 3); obtaining bismuth-mono-carboxylic acid complex in an aqueous solution by adding mono-carboxylic acid to trivalent bismuth ions, further precipitating the complex by adding an alkali in an aqueous solution, and separating the complex from the solution, and obtaining fine spherical bismuth oxide particles by firing the separated the complex (Patent Reference 4); vaporizing bismuth by heating at a high temperature, and supplying air to the vaporized bismuth (Patent Reference 5); obtaining bismuth nitrate solution by adding nitric acid and hydrogen peroxide to bismuth metal grains or powder, separating the precipitant by adding sodium hydroxide to the solution and heating, and washing, drying, classifying the separated precipitant, and obtaining bismuth oxide (Non-patent Reference 1); oxidizing fine particle bismuth metal by injecting air and heating at 850°-900° C. (Non-patent Reference 1); and other methods such as oxidizing bismuth metal at 750-800° C., pyrolyzing bismuth carbonate, and adding alkaline metal hydroxide to bismuth salt solution (Non-patent Reference 2).    Patent Reference 1: U.S. Pat. No. 1,318,336    Patent Reference 2: U.S. Pat. No. 1,354,806    Patent Reference 3: Japanese Patent Publication No. Sho 47-11335    Patent Reference 4: U.S. Pat. No. 4,873,073    Patent Reference 5: Japanese Patent Publication No. Sho 61-136922    Non-patent Reference 1: JOM; April 2002; 54, 4; ABI/INFORM Trade & Industry    Non-patent Reference 2: Ullman's Encyclopedia of Industrial Chemistry, Vol. 5, pp. 185-186